Process for the production of aliphatic dicarboxylic acids



March 12, 1940.

PROCESS FOR THE PRODUQTION 0F ALIPHATIC DICARBOXYLIC ACIDS Filed Oct. 8, 1938 Exn Gas 22 2/ (yd/c Condenser Compound 57orage Eeacfor fa i I L/7 3V s. H. MCALLISTER v 2,193,562 I Y Patented Mar. 12, 1940 UNITED STATES PATENT OFFlCE PROCESS FOR THE PRODUCTION OF "ALI- PHATIC DICARBOXYLIC ACIDS Sumner H. McAllister, Lafayette, Calii'., assignmto Shell Development (lompany san Francisco, Calif., a corporation of Delaware Application October 8,

16 Claims.

The present invention relatesto a process for v the production of aliphatic dicarboxylic acids. More particularly, the invention relates to a method -for the production of aliphatic diease with which the ring structure may be sprung.-

Such compounds as benzene, cyclohe'xane and cyclopentane are very diflicult to oxidize; Cyclic ketones, on the other hand, are readily oxidized to aliphatic dicarboxylic acids under mild conditions with a wide variety of oxidizing agents.

The cyclic compounds oxidized according to the present invention, i. e. cyclic alcohols, are intermediate between these two extremes. These compounds, although much harder to oxidize than the cyclic ketones, may be oxidized under fairly strong oxidizing conditions.

Compounds of this class, since they are relatively difficult to oxidize to .dicarboxylic acids,- require the use of a relatively strong oxidizing agent. One of the most suitable oxidizing agents for this purpose is nitric acid. Since nitric acid is a relatively expensive oxidizing agent, the economy of the nitric acid oxidation process is primarily dependent upon the consumption of nitric acid. In spite of the ready availability of the cyclic alcohols, the nitric acid oxidation of these compounds has not, in the past, been considered economical. This is due mainly to the fact that, under the conditions of oxidation hitherto employed, aconsiderable amount of dicarboxylic acids having a lesser number of carbon atoms than the material being oxidized was in- 4O variably produced. This not only resulted inan proposed, when applied to the oxidation of'cyclic 55 method whereby valuable aliphatic dicarboxylic an, Serial No. 234,030

acids may be prepared in a practicaland economical manner with better yields. from inexpensive and readily available cyclic compounds. Another object of the invention is to provide a method whereby maximum yields of aliphatic dicarboxylic acids containing the same number of carbon atoms as the cyclic compounds being oxidized may be obtained. A further object of the invention is to provide a nitric acid oxidation process for the production of dicarboxylic acids afiording the leastpossible consumption of nitric acid. Still a further object of the invention is to provide a practical method whereby aliphatic dicarboxylic acids containing an uneven number of carbon atoms-between the carboxyl groups may be prepared.

According to the present invention, these objects are accomplished by oxidizing cyclic alcohols with dilute nitric acid under certain carefully controlled conditions. In order that the prevailthe reaction system and the reaction mixture withdrawn at suitable rates in essentially a continuous manner.

In order to facilitate the understanding of my invention, reference may be had to the attached drawing which shows schematically one suitable assembly of apparatus by means of which the process maybe carried out. It is to be understood, however, that this simple assembly of apparatus is presented primarily to clarify the de-' scription and is not to be construed as limiting the invention. Referring to the drawing, I and 2 represent storage vessels for nitric acid and the cyclic compound to be oxidized; respectively. 3 represents a reaction vessel equipped with stir-' ring means 4, closed coil 5 for'controlling the temperature therein and a condenser l9. The cyclic alcohol is introduced continuously via a pipe 8 and control valve 9 into the reaction vessel containing dilute nitric acid maintained at the desired temperature by means of the closed coil. The desired concentration of nitric acid in the reaction. vessel is maintained by continuously adding stronger (fortifying) acid from storage .vessel l via a pipe l0 and control valve H. An agitating means 4 keeps the reacting mixture at the desired degree of agitation. The crude reaction mixturewhich consists essentially of the desired aliphatic dicarboxylic acid and-dilute nitric acid, is withdrawn continuously from the reaction vessel via a pipe I2 and control valve l8 and introduced into a fractionating column 6 provided with suitable heating means-,such as, 55

for example, a heating coil l3. The fractionating column is operated to take off unreacted nitric acid overhead via pipe 14, condenser 1 and pipe l5, leaving a concentrated nitric acid solution of the desired dicarboxylic acid. The distilledand condensed dilute nitric acid may be reconcentrated by known methods and returned to storage vessel l. The concentrated nitric acid solution of the desired carboxylic acid is withdrawn from the fractionating column near the bottom via a pipe l6 and control valve ll. dicarboxylic acid may be recovered and purified as more fully described below. The fixed gases produced by the reaction collect in the upper portion of the reaction vessel from where they are withdrawn through a pipe 20 and condenser l9. In passing through the condenser, any vapors of the unreacted alcohol and nitric acid are condensed and return to the reaction vessel. After being freed of more condensable constituents, the gases are withdrawn via pipe 2| and valve 22.

I have found that there are several factors which govern the consumption of nitric acid and the purity of the product (and hence the economy of the process). The most important single factor is the concentration of nitric acid in the reaction vessel. The temperature at which the oxidation is executed is a dependent variable of somewhat less importance. Other factors which affect the economy of the process to some extent are the presence or absence of a catalyst and the strength of the fortifying acid.

The concentration of nitric acid maintained in the reaction chamber, according to the present invention, is much lower than customarily used for the oxidation-of cyclic compounds. In general, the concentration of nitric acid is preferably maintained as low as possible, consistent with a satisfactory reaction rate. In order that the reaction proceed at an appreciable rate using the preferred very low nitric acid concentration, the reaction is preferably'executed at a fairly high temperature. In general, nitric acid concentrations ranging from about 8% to about 18% are preferred, although concentrations as low as 4% and as high as 24% may often be employed. Depending upon the concentrationof nitric acid maintained in the reaction vessel and somewhat upon the cyclic alcohol being oxidized, the temperature at which the oxidation is executed may range from about 50 C. to.130 C. or even higher. In general, temperatures between C, and 110 C. are preferred.

The dependency of the purity of the product and the consumption of nitric acid on the concentration of nitric acid maintained in the reaction vessel may be seen in the following table showing the results of comparable experiments on the oxidation "of cyclopentanol to glutaric acid at a temperature of100i1 C. in the presence of 0.3 gm. V205 per mol of cyclopentanol.

HNO; concentration, M01 HN co su ed 5 2E2 5; g

Percent M01 organic acid formed product In the case of the oxidation of the nonalkylated cyclic alcohols, using concentrations of nitric acid of about 10%, the preferred temperature is about 80 C. For higher acid con-- centrations the preferred temperature is some- The' desired.

what lower, while for less concentrated acid, the preferred temperature is somewhat higher. The dependency of the nitric acid consumption upon the temperature of oxidation is shown in the following table wherein the results of comparable experiments on the oxidation of cyclopentanol to glutaric using a nitric acid concentration of 10%:.3% and 0.3 gm. V205 per mol of cyclopentanol are given.

The preferred conditions of HNOa concentration and temperature may vary considerably within theabove broader limits depending upon the character of the cyclic alcohol being oxidized. In general, nonalkylated monohydric alcohols,

Such as cyclohexanol, cyclopentanol and substi-,

tuted derivatives thereof are preferably oxidized under more severe conditions, i. e., higher concentrations of nitric acid and/or higher temperatures, than their homologues containing alkyl sidechains. Cyclic alcohols containing long alkyl sidechains and those containing gem. alkyl groups, such as the butyl cyclohexanols, gem. dimethylcyclopentanol-3 and the like, in particular are preferably oxidized under the most mild conditions, i. e., acid concentrations of about 4%-8% and moderate temperatures.

The yield and purity of the product are somewhat dependent upon the'concentration of the fortifying acid continuously added to maintain the desired concentration in the reaction vessel. Concentrated acid, such as 50-70% nitric acid, has been found to be preferable to more dilute acids.

The present process may be satisfactorily carried out with or Without the aid of a catalyst. Any of the conventional oxidation catalysts such as the oxides and salts of the polyvalent metals are applicable and may be used if desired. The presence of a catalyst tends, in general, other factors remaining equal, to somewhat increase the yield and throughput capacity, to allow the use of somewhat lower temperatures, and to increase the ratio of Nz/NO in the exit gases, and in these respects is advantageous. On the other hand, the use of a catalyst tends, in general, to increase the amount of lower dicarboxylic acid produced. The effect of the presence of a catalyst is illustrated in the following table showing the results of a series of comparable experiments on the oxidation of cyclopentanol to glutaric acid in the presence of varying amounts of vanadium pentoxide. In these experiments, the nitric acid concentration was maintained at 9.3 i .7 And the temperature was maintained at 100i1 C.

Mol EN 0; consumed Percent ghb p Mol organic acid protaric acid in cent cyclopenta duced product yield The present process is of particular advantage when referring to a cyclic alcohol I mean a cyclic organic compound containing a carbinol group in a non-aromatic ring. As examples of suitable cyclic alcohols may be mentioned cyclobutanol, 2-methyl cyclobutanol, cyclopentanol, 3-methyl cyclopentanol, cyclohexanol, gem. di-

-methyl cyclohexanol, 3-nitro cyclohexanol, cyclohexenol A 3,5-dimethyl cyclohexanol, cycloheptanol, etc. As will be apparent, by the proper choice of cyclic alcohol a wide variety of aliphatic dicarboxylic acids may be produced.

While, in general, the process of the present invention. is executed at substantially atmospheric pressure, higherpressures are applicable and may often be used with advantage. Superatmospheric pressures, for example pressures up oxidation step. The concentration of the crude reaction mixture should not be effected under any substantial superatmospheric pressure.

The schematic arrangement of apparatus shown in the accompanying drawing illustrates a simple plant suitable for the oxidation under substantially atmospheric pressure. It is to be understood, however, that the process is not limited whatsoever to the use of any particular typ of apparatus. In such cases where pressure is to be employed, the assembly of apparatus shown diagrammatically in the attached drawing may be provided with pumps for introducing the reactantsinotshown) or, if desired, in place of .pumps, the reactants may be forced into the reaction vessel by'meansof gas pressure. Numerous equally suitable modifications and variations will be readily apparent to those skilled in the art. For instance, the reactants may be cirequipment having an acid-proof lining and equipment fabricated from corrosion resistant alloys such as duriron, etc., are applicable.

The crude reaction product withdrawn from the reaction vessel contains the desired aliphatic dicarboxylic acid in solution in dilute nitric acid along with any unoxidized cyclic alcohol and small amounts of other products of side reactions.

ture by any one of several methods, it has been found most advantageous to first concentrate.

thesolution to a point where a substantial quantity of the desired dicarboxylic acid may'be crystallized out upon cooling. This concentration may be most advantageously accomplished by conducting the crude reaction mixture continu-' ously, preferably without first cooling, into a suitable fractionating or stripping apparatus wherein'a substantial portion of the water'and nitric acid are removed. This method, I have found, has several advantages. The last traces of unreacted alcohol withdrawn with the re- Although the desired dicarboxylic acid may be recovered from the crude reaction mixaction mixture undergoes reaction during the flashing operation, and this appears to inhibit the further reaction of the dicarboxylic acids during the concentration step. Furthermore, it obviates the continuous loss of a .small amount of unreacted alcohol which could not be economically recovered. By subjecting the hot reaction mixture to an immediate fiash distillation, the time that the dicarboxylic acids are contacted with hot nitric acid is kept at a minimum, and

this helps to maintain high yields and a pure product; The saving of heat aiforded by this method is also of some commercial advantage. The dilute nitric acid solution recovered from such a concentration step may be easily reconcentrated and recycled.

The greater portion of the desired dicarboxylicacid may in most cases be separated from the hot concentrated solution from such a concentration step by simply cooling, whereby the dicarboxylic acid crystallizes out. The crystal. strike thus obtained, after separating from the mother. liquor, may be further purified, if desired, by drying at 30-50 0., preferably under diminished pressure, dissolving in a suitable solvent, such as acetone, decolorizing, if desired, by a charcoal treatment, and finally recovered pure from the solvent solution. Other conventional methods of purification, such as solvent extraction, etc., may also be used. The mother liquor from the first crystallization may still contain appreciable dicarboxylic acids. This solution may be further concentrated, if desired, and a second strike of dicarboxylic acid crystallized. The final liquor remaining after any number of concentrations and crystallizations, consists essentially of concentrated nitric acid, but still contains traces of dicarboxylic acid. This final liquor ma be reused in the process.

One of the most important uses of the aliphatic dicarboxylic acids is in the production of resins. When destined for this use, the dicarboxylic acid must be free from bodies tending to discolor the resin In a few cases, the dicarboxylic acids produced according to my process, although apthis purpose since they produced discolored resins. I have found, however, that this may be readily remedied by subjecting the dicarboxylic acid to a treatment with activated charcoal which has been degassed by heating in vacuo just prior to use. This treatment is not only helpful in reclaiming occasional ofi batches of dicarboxylic acid, but may also be used as a matter of course to produce a premium product.

According to thecommonly accepted teachings of the prior art, aliphatic dicarboxylic acids having an uneven number of carbon atoms between the carboxyl groups are much more difficult to prepare by oxidation than those having an even number of carbon atoms in the aliphatic chain. I have found, to my surprise, that in spite of the prevalent belief that dicarboxylic acids. having an uneven number of carbon atoms in the aliphatic chain can only be prepared-by oxidation with mild oxidizing agents and with poor yields, these acids may be prepared by the oxidation of the odd membered ring compounds of the'class in question, according to the process of my invention, with little difiiculty and with excellent yields. I

The present process has distinct advantages. According to the present above-described process, the desired dicarboxylicacid may be obtained inexcellentyields. The yields, in general, de-

, pearing colorless, have not proven suitable for pend upon the compound being oxidized and may vary considerably. The non-alkylated cyclic alcohols, in particular, give almost perfect yields when oxidized according to the present process. Thus, for example, the yields of dicarboxylic acids from these alcohols, in general, exceed and are often, under optimum conditions, about 98- 99%. The crude dicarboxylic acids preparedaccording to the present process contain, in general, only small amounts of monocarboxylic acids'and dicarboxylic acids of lower moleculariweight. The amount of dicarboxylic acids; having a lesser number of carbon atoms than the material being oxidized depends, of course, upon the material being oxidized, the prevailing conditions and upon the presence or absence of a catalyst, and may vary considerably.- In general, the amount of lower molecular Weight acids does not exceed 10% and, under favorable conditions, may be as low as 3%.

One of the main advantages of the present process is the economy with which it may be executed. As previously stated, the economy of any nitric acid oxidation process is, to a large extent, dependent upon the consumption of nitric acid. In the usual oxidation of organic compounds by nitric acid, the theoretical acid consumption is based on the complete reduction of the nitric acid to nitric oxide (NO). Thus, for example, the oxidation of a cyclic alcohol is represented in theory by the equation:

From which it is seen that the theoretical consumption of nitric acid is 2.67 mol per mol of aliphatic dicarboxylic acid produced. In the processes of the prior art, however, this theoretical consumption is not realized, theconsumptions in general being usually in the order of 5 to 7 mol. This is due to the fact that under the conditions employed, a substantial quantity of the nitric acid is reduced only to nitrogen peroxide (N204) while,

moreover, a considerable amount of nitric acid is consumed in undesirable side reactions.

When executing the oxidation according to the preferred embodiment of my invention, on the other hand, the consumption of nitric acid, based on the theoretical reduction to NO, is usually about, and in many cases, even below the theoretical. For example, cyclic alcohols may be oxidized to aliphatic dicarboxylic acids according to my process with a.nitricacid consumption of only 2.42 mol per mol of dicarboxylic acidproduced. The unusually low consumptions of nitric acid realized in the present process are due to the more complete reduction of the nitric-acid and to the fact that under the more favorable conditions used the side reactions are materially decreased. Analysis of the gaseous reaction products obtained when executing the reaction according to the present process have shown that under these more favorable conditions only small amounts of N204 are produced, while the greater proportion of the nitric acid is reduced to nitrous oxide (N20) and a considerable quantity is even reduced to elemental nitrogen (N2) The present process is of particular advantage for the production of dicarboxylic acids containwhich comprises continuously introducing cyclo-- pentanol into a nitric acid solution maintained at a temperature between 70 C, and 0., maintaining the concentration of said nitric acid solution between 8% and 18% by continuously introducing more concentrated nitric acid, continuously removing the crude reaction mixture and recovering glutaric acid therefrom.

2. A process for the production of aliphatic dicarboxylic acid which comprises continuously introducing a non-alkylated cyclic alcohol into a nitricacid solution maintained at a temperature between 70 C. and 110 C., maintaining the concentration of said nitric acid solution between 8% and 18% by continuously introducing more concentrated nitric acid, continuously removing the crude reaction mixture and'recovering aliphatic dicarboxylic acid therefrom.

3. A process for the production of aliphatic dicarboxylic acid which comprises continuously introducing-a non-alkylated cyclic alcohol into a nitric acid solution maintained at a temperature above 50 C., maintaining the concentration of said nitric acid solution between 8% and 18% by continuously introducing more concentrated nitric acid, continuously removing the crude reaction mixture and recovering aliphatic dicarboxylic acid therefrom.

4. A process for the production of aliphatic dicarboxylic acid which comprises continuously introducing a non-alkylated 'cyclic alcohol into a ,nitric acid solution maintained at a temperature above 50 C., maintaining the concentration of said nitric acid solution between 4% and 24% by continuously introducing more concentrated nitric acid, continuously removing the crude reaction mixture and recovering aliphatic dicar-- boxylic acid therefrom] 5. A process for the production of aliphatic dicarboxylic acid which comprises reacting a cyclic alcohol in the presence of an oxidation catalyst with'a nitric acid solution maintained at a temperature between 70 C, and 110 C., maintaining the concentration of said nitric acid solution between 8% and 18% by introducing more concentrated nitric acid, removing the crude reaction mixture and concentrating the same by distillation, crystallizing aliphatic dicarboxylic acid from the concentrated reaction mixture and finally purifying the aliphatic dicarboxylic acid by treatment with degassed charcoal.

6. A process for the production of aliphatic dicarboxylic acid which comprises reacting a cyclic alcohol in the presence of an oxidation catalyst with a nitric acid solution maintained at a temperature between 70 C. and 110 0.,

maintaining the concentration of said nitric acid solution between 8% and 18% by introducing more concentrated nitric acid, removing the crude reaction mixture and concentrating the same by distillation.

'7. A process for the production of aliphatic dicarboxylic acid which comprises reacting a cyclic centrated nitric acid, removing the crude reaction mixture and recovering aliphatic dicarboxylic acid therefrom.

8. A process for the production of aliphatic dicarboxylic acid which comprises reacting a cyclic alcohol in the presence of an oxidation catalyst with a nitric acid solution maintained at a temperature above50 C., maintaining the concentration of said nitric acid solution between 8%' and 18% by introducing more concentrated nitric acid, removing the crude reaction mixture and recovering aliphatic dicarboxylic acid therefrom.

and 24% by introducing more concentrated nitric acid, removing the crude reaction mixture and recovering aliphatic dicarboxylicacid therefrom.

10. A process for the production of aliphatic dicarboxylic acid which comprises reacting a cyclic alcohol having an uneven number of carbon-atoms in a carbocyclic chain with 'a nitric acid solution maintained at-a temperature above 50 C., maintaining the concentration of said nitric acid solution between 4% and 24% bycintroducing more concentrated acid, removing the crude reaction mixture and recovering aliphatic dicarboxylic acid therefrom. I

11. A process for the production of aliphatic dicarboxylic acid which comprises reacting a cyclic alcohol with a nitric acid solution maintained at a temperature above 50 C., maintaining the concentration of said nitric acid solution between 4%,.and 24% by introducing more con- 45 centrated nitric acid, removing the crude reaction mixture and recoveringv aliphatic dicarboxylic acid therefrom.

12. In a process for the production of glutaric acid the step of continuously oxidizing cyclopentanol with a nitric acid solution maintained between 4% and 24% nitric acid concentration and at a temperature between 70 C. and 110 C.

13. In a process for the production of aliphatic dicarboxylic acid the step of continuously oxidizing a cyclic alcohol having an uneven number of carbon atoms in a carbocyclic chain in a nitric acid solution maintained between 4% and 24% nitric acid concentration and a temperature between 70 C. and 110 C.

' 14. In a process for the production of aliphatic dicarboxylic acid the step of continuously oxidizing a cyclic alcohol in nitric acid solution maintained between 4% and 24% nitric acid concentration and at a temperature between 70 C. and 110 C. I

15. In a process for the production of an aliphatic dicarbfoxylic acid which includes the introduction of an alicyclic alcohol into a nitric acid-containing solution maintained at a temperature between 70 C. and 105 0., the step of maintaining the concentration of the nitric acid in said solution between 5% and 14% substantially during-the reaction between the nitric acid and the a'licyclic alcohol by the addition of fortiiying nitric acid as required whereby nitric acid is economically consumed. v i

16. In a process for the production of an aliphatic dicarboxylic acid, the steps of forming a body of reaction solution containing dilute nitric acid in a reaction zone, feeding an alicyclic alco" hol to said bodyv of reaction solution, introducing also fortii'ying nitric acid into said reaction zone at a rate suflicient to maintain oxidizing conditions and dilute nitric acid in said reaction zone, and withdrawing the reaction mixture containing formed aliphatic dicarboxylic acid from said reaction zone at a rate correlated to the rate of ad- .ditions to said zone so as to maintain a body of reaction solution in said zone.

SUMNER 1H. McALLIsrEa; 

